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TECHNITOPICS
Page 65
Page 66
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By P.A. C. BROCK1NGTON, A Ng I Meet' E
AIR RESISTANCE DISCUSS goods vehicles with the average C-licensee or haulier, and I'll guarantee that most technical subjects will come up before he mentions wind resistance —if, indeed, he even thinks about it. I suppose that it is rather natural to connect wind drag with streamlined and high-powered sports and racing cars. Yet it has quite an effect on a laden goods vehicle.
A typical vehicle manufacturer might perhaps add, as an afterthought, that if someone could tell him what windage losses were with different types of load, he would discuss the subject with his design staff, particularly with regard to continuous high-speed running on the motorways. Unfortunately, no one can provide data on the subject, at least in this country; but the back-room boys at MIRA have obtained the drag, sideforce and yawing-movement coefficients of more than 40 cars (and one Minibus) and take the subject very seriously. They have established, for example, that the addition of a loaded roof-rack to a small saloon car adds 80 per cent to the Overall drag, the drag coefficient being increased by some 40 per cent. I am assured that, with a little encouragement, they would be fully prepared to make wind-tunnel tests of any scale-model lorry or p.s.v. that a member company might provide, but no commercial vehicle manufacturer appears to be interested.
It is highly doubtful whether a lorry or bus could he streamlined to give a worth-while gain, except possibly in special cases, without encroaching on the payload, but employing transmission ratios that offset a high aerodynamic drag coefficient to the maximum possible extent is an exercise that could be undertaken immediately the coefficient were determined, and its availability would also enable the most appropriate power unit to be employed.
For motorway service,. the yardstick of optimum-performance-with-economy of a laden vehicle is fairly clear cut and recognized by the top authorities, notably
in America. Power output and ratio availability should be matched to the overall load factor (including aerodynamic drag) to provide an appropriate speed on the level when the engine is operating at 70-80 per cent load. The " appropriate speed is normally about 50-60 m.p.h. unless the accelerated tyre wear and fuel consumption associated with higher speeds are justified by the time saved in this way.
In due course, motorways running will become much more organized and specialized than it is today with regard to vehicle suitability and driving techniques. Unless the aerodynamic drag coefficient is known, accurate matching will be impossible. In a known case, installing a 6-ft. headboard above the cab of an 8-ton platform vehicle reduced its top speed from 64 to 48 m.p.h.
Whilst it is impossible to dogmatize regarding the possible value of the odd fin, contoured corner or re-shaped windscreen, the greatest scope for streamlining is almost certainly provided by the articulated outfit in the form of cowling in the space between the cab and the body, which increases drag by creating air turbulence. It could well be, however, that a simple type of canopy sloping down from the cab roof to the tailboard of a lorry carrying a flat load would reduce drag very considerably.
MORE AIR FOR CONSTANT HORSEPOWER
Turbocharge a diesel engine, if necessary with a low-pressure and high-pressure turbocharger in series, and use a governor that supplies a decreasing amount of fuel per working stroke as the speed goes up, and you have a constant horsepower engine. Ifs as simple as that according to W. H. Dorman and Co. Ltd.. of Stafford, and, with the help of CAV, they have proved it. Equipped with a CAV Type 24 and a Type 78 turbocharger and a CAV falling-torque governor, a Dorman six-cylinder 9-57-litre (four-stroke) unit develops around 120 b.h.p. from 1,000 r.p.m, to the maximum speed of 1,800 r.p.m.
Customers of the company who make earth-moving machinery are not, however, as yet interested in the project, possibly because the naturally aspirated version of the engine has an output of 130 b.b.p at the higher speed and the standard turbocharger unit produces about 210 b.h.p. But both Dorman and CAV think that the system is a far better way of achieving a constant horsepower than employing a differentially driven blower as Perkins do with their DDE unit.
Comparing the two systems is a complex exercise because Perkins raise the power of the engine at the top end with a differentially driven supercharger and increase supercharger pressure at the lower end to give a falling-torque curve. Dorman chops off power at the top end and adds to torque at the lower end, so the power-to-weight ratio is about double that of a standard turbocharged engine— which would not be acceptable to a vehicle maker or operator.
But this is by the way. The system offers a means, without raising the peak power, of eliminating gear changing (thereby improving acceleration and gradability) for a very substantial part of the load-speed range and possibly of saving weight by reducing the number of ratios required to two or three, the need to cater for the higher low-speed torque being a factor that would have to be taken into account.
Because the cylinders receive the full output of the turbochargers at higher r.p.m., surplus air is available for any increase in power up to the limit of, say, 60 per cent. In the case of a typical I50-b.h.p. engine it might be expedient, for example, to raise the peak output to 170 b.h.p. in addition to increasing the b.h.p. from 90 at 1,000 r.p.m. to 150 at the lower speed, which would certainly not incur acute thermal loading problems. lithe engine would accept an increase to more than 200 b.h.p. and were fitted in a larger vehicle or to provide a much higher top speed, the torque back-up at lower r.p.m. would still be very useful to have.
Incidentally, the Dorman constanthorsepower engine provides a minimum specific fuel consumption of better than 0-37 lb./b.h.p./hr, between 1.200 r.p.m. and 1.400 r.p.m. B.m.e.p. of the engine at 1.000 r.p.m. is 160 p.s.i. compared with a b.m.e.p. of 102 p.s.i. produced by the naturally aspirated 130-b.h.p. unit and 155 p.s.i. of the standard turbocharged engine. CAV believes that it would he possible to turbocharge a 150-b.h.p.
c8 vehicle engine to give 200 b.h.p. in the range 1,000-1,800 r.p.m.
A substantial proportion of Dorman turbocharged engines are equipped with air-to-air intercoolers which enable output to increase by some 20 per cent Without an increase in peak cylinder pressure. Intercooling also reduces specific fuel consumption appreciably and the mean temperatures of the pistons and exhaust valves. Performancevvise, this is something for nothing, and it is .pertinent that Perkins claim a reduction in exhaustvalve temperature of 50°C by charge cooling, combined with an increase in power of 17 per cent.
AIR IN THE RIGHT QUANTITY
The water that cools the cylinders is cooled by air, so why bother with a lot of plumbing tb use water? When water boils it carries heat away more rapidly than when it's not boiling, so it gives a big reserve of cooling potential, as well as acting as a sound-dampening medium: and if the radiator fan fails, the water has to boil away before things become critical, If air could be circulated round the finning of an air-cooled engine in proportion to the output and there was nothing to go wrong mechanically, the unit could be uprated and air-cooling might come into its own in a big way for commercial vehicle use.
Exhaust ejector cooling could be the answer, and judging by a recent article by Mr. A. V. Zhlobich in the journal
Avtmobit maya Promlishennost the Russians are hopeful that a successful ejector system could be developed. Going back to Julius Mockeries book, "AirCooled Motor Engines ", published in 1961, the Tatra works of .Czechoslovakia consider silencing difficulties are the only major snag.
Ejector cooling is simple in principle. The exhaust gas is discharged in a duct forming the outlet of cowling round the engine, and this draws air across the finning approximately in proportion to engine output. Mr. Zhlobich's tests have shown that separate ejectors for each cylinder or pairs of cylinders offer improved efficiency by exploiting pulse energy (this conflicts with the Tatra tests) and he observes that applying the principle to diesel engines is more rewarding than cooling petrol engines by the ejector method. It is pertinent that maintaining the optimum running temperature of a diesel at low loads is more difficult than controlling the temperature of a petrol engine because the diesel offers a higher gain in thermal efficiency at reduced outputs, and a conventional cooling system has a higher over-cooling potential.
Claims for ejector cooling Ore, however, discounted by Mr. Eric Kellett, of CAV, who states that ejector pumping power would be inadequate—but maybe he has not fully investigated the potential of pulse energy. Mr. Kellett does not believe the air-cooled diesel has a future, but considers that a fan driven by an exhaust turbine could provide efficient cooling.
In contrast with an ejector system, a turbine might go wrong mechanically, and failure could be dangerous; but, as would be the case with the ejector, it would absorb negligible power.
Now employed in the Motor Vehicle Research Institute, Prague, Mr, Mackerle is working on a completely new type of vehicle drive (according to the " SNTL Technical Digest ") the potential of which would appear to be limited to slowmoving vehicles, but the system is so revolutionary that it defies forecasts by known yardsticks. Models have operated successfully on uneven ground and steep slopes. Greatly reduced transmission losses are claimed.
The rubber tyres of the driving-axle wheels are of the multi-chambered type (with eight or more chambers) and the pressure in each is caused to fluctuate in rotational order. In effect the chambers act as legs, a rise in pressure in the chamber behind the wheel axis causing the wheel to revolve in a forward direction,